Silicon (Si) is the most widely used semiconductor material, and has been for many years. Due to intense commercial interest and resulting research and development, Si device technology has reached an advanced level, and in fact, many believe that silicon power deices are approaching the theoretical maximum power limit predicted for this material. Further refinements in this material are not likely to yield substantial improvements in performance, and as a result, development efforts have shifted in focus to the development of other wide band gap semiconductors as replacements for silicon.
Silicon carbide (SiC) has many desirable properties for high voltage, high frequency and high temperature applications. More particularly, SiC has a large critical electric field (10 times higher than that of Si), a large bandgap (3 times that of Si), a large thermal conductivity (4 times that of Si) and a large electron saturation velocity (twice that of Si). These properties support the theory that SiC will excel over conventional power device applications, such as MOSFETs, SiC n-channel enhancement mode MOSFETs, and SiC diodes such as a merged PIN Schottky (MPS) or a junction barrier Schottky diode(JBS).
Although SiC based semiconductor devices thus provide many advantageous properties as compared to Si devices, the material properties of SiC can make it more difficult to process than Si. As a result, and generally speaking, those of ordinary skill in the art of semiconductor processing would not expect processes useful in the fabrication of an Si device to be useful in the fabrication of an SiC device, and vice versa. As but one example, SiC is more chemically inert than Si and so any manufacturing processes relying on the reactivity of the substrate, such as etching or chemical mechanical polishing or planarization (CMP), will necessarily be different for each material.
One example of devices advantageously based upon SiC substrates are the metal oxide semiconductor field effect transistors (MOSFETS). SiC MOSFETS may typically be processed with ion implantation and/or epitaxial growth for the deposition of features on the substrate. CMP may subsequently be utilized to planarize the ‘bumpy’ surface that can result from ion implantation, or to remove any unwanted epitaxial grown material from designated areas in order to leave the desired feature on the substrate. With respect to the latter, CMP is preferable to either gas or liquid phase etching as these methods, relying only on chemical removal, may not provide commercially acceptable removal when applied to the relatively inert SiC. Either vertical or lateral MOSFETS may also typically comprise buried channels, which are desirably left undisturbed by any CMP of the device surface.
Determining the endpoint of a CMP polish is challenging with any kind of material, and whatever the material, additional challenges may be presented if the surface being polished is non-planar and/or comprises buried channels. In any case, removing too much or too little material can render the resulting device non-functional. Typically, the endpoint of a CMP polish is determined by back calculating an appropriate etch time given the known etch rate of the polish protocol and the material being polished. Once the calculated time has been reached, the device is removed from the process, cleaned and the thickness of the remaining layer measured, typically via optical imaging.
Unfortunately, these methods may provide less than optimal results. Firstly, the etch rate may actually fluctuate during the process due to even slight fluctuations in any of a number of conditions, in which case, the calculated time will be incorrect. Additionally, the use of etch rate to calculate a process time may be suboptimal in applications where a non-planar surface is desirably being treated. Secondly, even though optical measurements are typically very accurate, some optical measurement techniques require destruction of the sample and may not be capable of accurately measuring small changes.
It would thus be desirable to provide improved methods for dimension profiling of SiC devices. Any such method would desirably not detrimentally impact either the process, e.g., via the addition of time, cost or safety concerns, or the device, e.g., by the incorporation of undesirable components for use in detection that may detrimentally affect device performance.